Fighting Fire With Fire to Save Idaho Resort

Firefighters Use Path of 2007 Wildfire to Help Temper Current Blaze

Firefighters are turning to an old ally in their fight to save Sun Valley, Idaho, from a wildfire: another fire.

Flames from the so-called Beaver Creek fire have been bearing down on part of the famed Idaho resort community since Aug. 7, but they started fizzling after hitting an area previously burned in the Castle Rock fire of 2007.

While Ketchum, Idaho, and other communities that make up Sun Valley remained under threat Wednesday, firefighters on the scene of the blaze that has consumed more than 108,000 acres of timber and brush said having the previous fire has kept the resort area more protected than it would have been.

Although 570 homes remained under an evacuation order Wednesday, with another 6,775 on alert to be ready to leave, the fire had burned only one home and a handful of other structures and caused no major injuries.

"Without Castle Rock, we would have had a much larger [fire] front to deal with, and it probably would have made it into Ketchum," said Wayne Patterson, a spokesman for an interagency command center in Hailey, Idaho.

The Sun Valley fire is one of 51 large blazes being fought around the U.S., mostly in the West, according to the National Interagency Fire Center in Boise, Idaho. Although the 3.5 million of total acreage burned in wildfires so far this year is running below a 10-year average of 5 million, fire officials said the infernos this year have at times been unusually severe—pushing firefighting costs to $1.2 billion, compared with $1.9 billion for all of last year, according to the center. With the fire season expected to last at least two more months in places like California, those cost are likely to rise.

Across the West, firefighters are using similar firebreaks from past fires where they can to slow new fires. In general, fire experts say, the older fires remove dead and dying trees that have clogged many forests, due to factors including drought and infestation of wood-eating insects—in essence, performing the thinning work that many experts say is needed to make the West's forests less combustible.

In Montana's backcountry, a series of fires last year were slowed when they ran into previously burned areas, officials said.

In Yellowstone National Park, firefighters used a burn area from 2009 to help keep a new blaze in 2010 from spreading. In Southern California, past fire scars helped firefighters quell some of the numerous conflagrations that flared up there in October 2007, said Dennis Mathisen, spokesman for the California Department of Forestry and Fire Protection. More than 1,500 homes were destroyed in those firestorms."We try to box the fire in using boundaries that make sense, such as a large street, a river or a previous burn area," Mr. Mathisen said.

Both the Castle Rock and Beaver Creek fires started when lightning bore down with such intensity that many residents worried the community known for its wealth and high-profile homeowners, such as actors Tom Hanks and Bruce Willis, would be overrun. The 42,000-acre fire in 2007 ended up causing little property damage, but left a buffer of scorched land on the western side of the community between other parts of the Sawtooth National Forest that remain overgrown, said Mr. Patterson, who is also a firefighter.

The prior burn "has been able to protect most of Sun Valley," said Laura Jorgenson, front desk clerk at the Best Western Plus Kentwood Lodge in Ketchum. "I would hate to think what this would have done without that burn."